Catalysts are widely employed in the petrochemical industry, particularly in reforming operations, for converting normal paraffins and other less desired constituents of a hydrocarbon feedstock to desirable aromatic compounds. The catalysts employed for these purposes typically include a substrate and one or more Group-VIII metals, most typically platinum, dispersed on the substrate, and may also include a binder for the substrate to form industrially useful aggregates.
Reforming catalysts may be prepared by depositing salts of the catalytic metal on the substrate and then calcining the substrate with the deposited salts in an oxidizing atmosphere to convert the metals to oxides and/or complexes including the metals, oxygen and other components, typically halogens such as chlorine. The catalyst in this oxidized condition ordinarily is loaded into a reactor and contacted with hydrogen at an elevated temperature to reduce the metallic oxides or oxide complexes to the free metal or active state. In a typical reforming process, the hydrocarbon feedstock is passed over the catalyst together with hydrogen at an elevated temperature. Under these conditions, some of the paraffins in the feedstock react to form the desired aromatic products. Other important hydrocarbon reactions occurring during reforming include the dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to aromatics, isomerization of normal paraffins to isoparaffins, dealkylation of alkylbenzenes, and hydrocracking. Hydrocracking reactions (which are to be avoided) produce light gaseous hydrocarbons, e.g., methane, ethane, propane and butane. After the catalyst has been so employed for a prolonged period, it typically loses some activity via the twin processes of coking and catalytic metal agglomeration. The catalysts may be regenerated by processes which include exposing the catalyst to oxygen at an elevated temperature to burn off coke deposits accumulated on the catalyst during its exposure to the feedstock. The coke burning operation ordinarily also causes some agglomeration of the catalytic metal into relatively large particles.
This agglomeration may be reversed, and the metal redispersed, by exposing the catalyst at an elevated temperature to a gas comprising oxygen, a source of a halogen and, typically, some water vapor to form oxyhalides or halides of the metal. The term "oxychloride" obviously refers to an oxyhalide in which the halide is chlorine. After redispersion, the catalyst may then be treated with a hydrogen-containing gas to reduce the metal to the free metallic or active state.
The dispersion procedure may be employed before the catalyst is exposed to the feedstock; that is to say, before the catalyst is first reduced, to enhance the dispersion of the metal on the catalyst. As used in this disclosure with reference to catalyst treatment procedures, the terms "activation" and "activating" refer broadly to procedures for bringing the catalyst to an active state, regardless of whether or not the catalyst has previously been employed to treat a feedstock, whereas the terms "regeneration" and "regenerating" refer specifically to procedures employed for bringing previously used catalyst to an active state.
The nature of the substrate and the distribution of the metal on and in that substrate can profoundly affect the performance of the catalyst. Catalysts incorporating a zeolite base, notably a type L zeolite base, may be employed in reforming operations as set forth in U.S. Pat. No. 4,104,320. A "type L zeolite" is a zeolite having a particular crystal structure as set out below. Type L zeolite-based catalysts provide better selectivity to C.sub.6 -C.sub.8 aromatics than do other commonly used commercial reforming catalysts. As disclosed in copending, commonly assigned U.S. patent application No. 550,951, filed Nov. 10, 1983, (now abandoned) the activity and selectivity of a type L zeolite based catalyst are markedly improved by dispersing the catalytic metal throughout the pores or channels of the zeolite. The '951 application discloses various regeneration procedures which are said to result in good dispersion of metal on the zeolite L substrate. Excellent catalysts based on type L zeolites have become increasingly important in reforming operations with the advent of the particular improved type L zeolites with a cylindrical morphology as described in U.S. Pat. No. 4,544,539. These zeolites provide, inter alia, increased catalyst life in conjunction with the other benefits of type L zeolite catalyst substrates.
Although the procedures disclosed in the aforementioned U.S. patent application Ser. No. 550,951 provide good results, further improvement is desirable.
The '951 application does not recognize the benefits attributable to a reduction step practiced at a temperature sufficiently lower than the chlorination or oxychlorination step; the benefits attainable by cooling the catalyst in the presence of oxygen before that reduction.